Elders don't try and contact the person if they have moved far away. But if they live in the territory and they haven't moved............there is no excuse.
There are instructions to JWs who meet a person from door to door who identifies themselves as DF'd or DA'd, to take their name and address and give it to the elders. The elders are supposed to try and contact them. JW family members will sometimes send the name and address of a DF'd family member to the elders in that territory.
Those are some ways the elders get the addresses of those who are DF'd or DA'd.
Actually, the elders are usually glad when you move out of their territory since they are no longer held responsible for you by the WTS.